Facebook’s Michael Adkins, a Messaging Integrity Engineer, explained what led to the shift of STARTTLS encryption:

We previously reported that only 28.6% of our outbound notification emails were successfully encrypted and passed strict certificate validation (58% if you count opportunistic encryption). Since STARTTLS encryption requires both sides to deploy it, we encouraged others to take the next step. As a result of recent changes by major providers, most notably Microsoft and Yahoo, 95% of our notification emails are now successfully encrypted with both Perfect Forward Secrecy and strict certificate validation.